# Where paper's dry strength comes from

Dry strength as hydrogen bonding across a limited bonded area, why water beats those bonds at their own chemistry, and what starches, cellulosic polymers and enzymes actually contribute.

Paper is held together almost entirely by hydrogen bonds that water outbids one for one — and every dry-strength additive either adds bond sites or enlarges the area where bonds can form.

Source: https://en.bioecon.ru/docs/forestry-biomaterials/cellulose-lignin/paper-bio-dry-strength-agents/
Updated: 2026-09-07



Dry paper is an unusual solid: its fibres are not glued, melted or welded to each other. The sheet owes its existence to the water that made it — as the wet web dries, the receding menisci draw fibres into conformal contact, and the hydroxyl groups on facing surfaces bond. Dry strength therefore lives in two quantities: how much area the fibres actually share, and how many hydrogen bonds survive per unit of that area. Every strength additive on the market works on one of the two.

## Bonded area, not glue

The classical theory of paper tensile strength, published by Page in 1969, treats the sheet exactly this way: strength rises with the shared (bonded) area between fibres and with the strength of the bonds across it, and falls when fibres themselves fail. Bonded area is set by geometry — fibre flexibility and collapse, fibrillation of the surface, the fines that pack the contacts. Refining raises it by flattening and fibrillating fibres, which is why refining has always been the brute-force strength tool, paid for in drainage and energy. Bond strength per area is mostly the density of accessible hydroxyl pairs; fillers interrupt contacts, which is why filling a sheet cheapens it twice, in material and in network.

## Why water wins

Water is simply a better hydrogen-bond partner than another hydroxyl: one molecule offers two donors and two acceptors. It sorbs into the interfacial zone, hydrates the bonded hydroxyls and pushes the surfaces apart, and dry tensile collapses to a small fraction of itself. Once-dried fibres never fully recover this capacity — the [hornification](../nanocellulose-biomaterials/) that penalises recycled furnish is the same loss seen from the pulp side. Worth stating what would be required to stop it: covalent crosslinks that water cannot swap, which is real wet-strength chemistry and a different product class. Dry-strength agents deliberately do not build those; the moment paper must stay strong wet, the mechanism changes.

## What the additives actually add

Three honest routes, all answering to the same two quantities. The first adds bonds: cationic starch, carboxymethyl cellulose or chitosan, retained on the negatively charged fibre surfaces, deposits polymer exactly in the contact zones — extra hydroxyl-bearing (or amino-bearing) chains standing in for the ones the refining could not bring together. The second adds area: micro- and nanofibrillated cellulose spans the gaps between fibres and acts as a bridge, converting non-contact into contact, as do the extra fibrils refining creates. The third makes area cheaper: low doses of cellulase modify the fibre surface so that a given level of fibrillation costs less refining energy, and other enzyme programs strip weak boundary layers so bonds form on sound cellulose. What none of them can do is make the resulting bonds waterproof — the added polymer hydrogen-bonds, and water outbids it like all the rest. Overdose is real: polymer that ends up in the water instead of at the interfaces hurts drainage and retention, which is why the additive is dosed by what the wet end retains, not by what the drum holds. The limiting quantity of the whole subject is shared area — and how much of it water is occupying at the moment you test.